Signal transduction is the process by which extracellular signals are transduced into a cell, and thereby impact upon process such as cell proliferation and differentiation. Tyrosine phosphorylation is a common mechanism for transducing a signal between signaling factors. Tyrosine kinases are enzymes which are capable of phosphorylating peptides and polypeptides at tyrosine residues. A subset of tyrosine kinases is also capable of dimerizing and thereby phosphorylating itself. Phosphatases are enzymes, which can optionally be receptors as well, capable of dephosphorylating a molecule such as a signaling factor. Uncontrolled tyrosine kinase activation has been implicated in the increased proliferation of cancerous cells.
In addition to tyrosine kinases and phosphatases, signal transduction pathways also use adaptor proteins. Adaptor proteins are intracellular proteins that act as intermediary proteins. These proteins allow for the association of other, usually signaling, proteins which would not otherwise be capable of interacting with each other. Adaptor proteins commonly do not have an activity other than their “adaptor” function, although they may act as substrates for other proteins such as, for example, kinases. Many adaptor proteins share conserved protein interaction domains such as, for example, Src homology (SH) domains. SH2 domains recognize and bind to phosphorylated tyrosine residues.
Abnormal signal transduction has been associated with a variety of disorders. A number of oncogenes are mutant variants of normally occurring signaling factors, including tyrosine kinases. One strategy for preventing or treating such disorders is to interfere with the unregulated or increased signal transduction which is thought to occur in these disorders.